What are some cues that my 4mo old is overstimulated?

Extreme Behavior. When a baby is overstimulated, they have trouble with normal routines, such as difficulty feeding. Also their sleep patterns are disrupted, thus they appear hungry and won't eat, sleepy and can not fall asleep.This will resolve when normal routines resume and less overt stimulation occurs.
Fussiness. Increased fussiness and trouble sleeping can be signs that your infant is overstimulated. But you have to make that assessment in the appropriate context because those can also be symptoms of other things (infection, pain, soiled diaper, etc.).